How does the proton get its spin?

At a meeting this week of the American Physical Society in Washington, MIT Associate Professor of Physics Bernd Surrow reported on new results from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) that provide a better understanding of the internal structure of the proton, the basic building block of all nuclei.

The world’s only polarized proton collider, at Brookhaven National Laboratory in Upton, N.Y., RHIC is used by MIT physicists to understand how the proton gets its spin, a fundamental quantum mechanical property (spin manifests itself as an intrinsic magnetic field, a property that is the basis of magnetic resonance imaging, or MRI). In 2009, spin-polarized protons were collided in RHIC at a record high center-of-mass energy of 500 giga electron volts (GeV). At this high energy — an energy 250 times the mass of the two individual protons making the collision — the protons are moving essentially at the speed of light and the quarks inside the proton are able to “see” each other at a resolution that is very small compared to the size of the proton. This allows scientists to study the proton’s internal structure.